Opacity of clay films

ABSTRACT

A small amount of a bismuth compound is incorporated with kaolin clay during the processing of the clay. The clay product is employed in an aqueous coating composition adapted for coating book or magazine paper in order to increase the opacity of the resulting clay film.

United States Patent 91 Zentz, Jr. June 11, 1974 ()PACITY 0F CLAY FILMS Prima Examiner-Delbert E. Gantz 751 tzWl E. t .ll,N.. TY I or ll lam Zen Jr Se m J Assistant Examiner-J. V. Howard Assigneer ngel Minerals & Chem ca s Attorney, Agent, or Firm-Melvin C. Flim; Inez L.

Corporation, Woodbridge, NJ. Mos ll TTM'TTMWMTM [22] Filed: July 10, 1972 [21] Appl. No.: 270,498 [57] ABSTRACT A small amount of a bismuth compound is incorpo- [52] us. Cl. .4 106/288 B, 106/72, 106/308 B rated with kaolin y during the Processing of the [51] int. Cl. C08h 17/06, C04b 33/04 Clay. e ay p o uct is employed in an aqueous [58] Field 01 Search lO6/288 B, 309, 72 ating mp siti n adapted for coating book or magazine paper in order to increase the opacity of the re- [56] References Cited sulting Clay UNITED STATES PATENTS l0/l959 Ruskin 106/72 5 Claims, No Drawings OPACITY OF CLAY FILMS BACKGROUND OF THE INVENTION The paper rawstock used in making publication grade paper lacks the smoothness, gloss and whiteness required for high quality printing. Especially because of the lack of smoothness the paper is incapable of registering accurately the fine dots used in halftone printing.

The most economicalmethod for providing a smooth white surface on paper intended for printing is to coat the paper with a thin film of clay particles. This is accomplished by applying to the rawstock an aqueous coating color" containing clay pigment, pigment dispersant (deflocculating agent) and suitable adhesive.

Given two or more clay pigments which provide comparable gloss, whiteness, printing smoothness and ink receptivity at similar cost, the coater will normally select the clay pigment which provides the greatest film opacity. By opting for the pigment that produces the most opaque coated sheet, the paper producer will generally obtain products which will exhibit less strikethrough of printed matter from the reverse side of the sheet. Further, there will be less show-through of printed matter in a pile of printed sheets.

It is known that the opacifying or hiding power of kaolin clay varies with the particle size and shape of the clay particles. Moreover, certain processing, especially some grinding techniques, results in kaolin clay pigments of improved opacifying power. It is generally believed, however, that the opacifying power of kaolin pigments is ultimately limited by the refractive index of the clay. Therefore, it has been considered necessary to blend clay pigments with more costly pigments having higher refractive indices in order to increase significantly the opacity of clay coatings.

PRIOR ART compound with a clay has been the one in which such I compound is used to increase the gel strength of aqueous suspensions of swelling bentonite clay.

THE INVENTION Briefly stated, in accordance with my invention a small amount of a bismuth (+3) salt is mixed with kaolin clay pigment in the presence of water prior to incorporating the clay pigment in a neutral or alkaline aqueous coating composition suitable for coating paper or for water-based paints.

Paper sheet or board coated with a clay coating composition of the present invention has greater hiding power (opacity) than it would have in the absence of the added bismuth compound. By the practice of this invention, clay films having unusually high scattering coefficients have been obtained. 'The bismuth compound which is added to the clay does not have an adverse effect on the rheology of the clay.

DETAILED DESCRIPTION Suitable bismuth salts are those which are soluble in acid solution and include bismuth (+3), nitrate, chloride, bromide, sulfate and carbonate. These salts form oxysalts in water, in accordance with the following representative equation:

BiCl H O BiOCl 2H 2Cl I The corresponding oxysalts of the abovementioned bismuth (+3) salts may be used. As indicated by the equation, such oxysalts will revert to normal salts when incorporated with clay at a sufficiently high hydrogen ion concentration. During subsequent dispersion, wherein the pH of the clay suspension is increased, oxysalts would be formed. The bismuth salt may be formed in situ by adding bismuth hydroxide to. an aqueous acidic suspension of the clay.

Recommended is the use of bismuth compound in amount within the range of 0.05 to 1 percent, preferably 0.1 to 0.5 percent, of the moisture-free weight of the clay. When insufficient bismuth salt is used, the effect on opacity may be minimal. On the other hand, the additive may impair other desired properties of the clay, especially rheology and'ink absorption, when used in excessive quantity. A suitable quantity may be determined by a screening process using black glass plates as the substrate for thefinished coatings and determining the scattering coefficients of coatings formulated with various proportions of additives. A publication setting forth details of the procedure appears hereinafter.

In one especially preferred embodiment of the invention, the bismuth salt is used with a specific type of kaolin clay which ischaracterized by extremely fine particle size and outstanding glossing properties. The clay is frequently described in the clay art as hard clay. When used in conventional coating color formulations, films containing the hard clay are glossier than those obtained with conventional soft kaolin clay coating pigments. However, the films with the hard clay lack the opacity obtainable with soft clay. When a suitable bismuth salt is used with a hard clay pigment, the coated films may be comparable in opacity to films obtained with the soft clays and they are frequently even glossier than the high gloss films obtained when hard clay is used in a conventional formulation. Thus, novel coatings featuring the combination of outstanding gloss and opacity are provided.

In another preferred embodiment, the bismuth compound is used with ground or mechanically delaminated kaolin clay which normally has high opacity. The use of the bismuth compound may improve film opacity to the extent that coat weight may be reduced.

,In another preferred embodiment, thebismuth compound is employed with No. 2 coating clays (kaolin pigments'containing a greater percentage of plus 2 micron particles than No. l coating clays) to provide coatings having properties, especially opacity, comparable to those obtained with the finer No. l coating clay.

In another embodiment of the invention, the bismuth compound is incorporated with a high brightness fine size fraction of soft kaolin clay to achieve, in effect, a coated film having the hiding power comparable to that of a film including a pigment having a higher refractive index than kaolin clay.

In carrying out my invention, the bismuth salt is mixed with the clay pigment,-preferably in the presence of water, before the pigment is deflocculatedcompletely by addition of a dispersing agent such as a sodium condensed phosphate salt and then mixed with a suitable binder system to form a coating composition.

In practicing the invention, the conventional wetprocessing may be modified merely by adding the bismuth compound as a solid or aqueous solution to a moist acidic filter cake of bleached kaolin clay before a condensed phosphate or other deflocculating agent is incorporated. The processing after addition of the bismuth compound to the cake will vary, depending upon whether the clay is to be supplied in acid or predispcrsed form. As another possibility, the bismuth compound can be added to a bleached kaolin clay suspension before the suspension is filtered.

An acid clay pigment may be obtained by drying the treated filter cake, grinding and pulverizing the dry mixture. in use the acid pigment containing the additive would be prepared into a deflocculated slurry by mixing the pigment with water and a condensed phosphate dispersing agent.

The usual practice of incorporating an optimum proportion of condensed phosphate dispersant may be followed in practicing this invention. An optimum proportion of dispersant is the minimum quantity that results in a clay-water slip of desired solids content which possesses minimum high shear viscosity. The high shear viscosity is usually measured by a Hercules or Hagan instrument. Sodium, potassium and ammonium coneensed phosphate salts may be used. For reasons of economy, the sodium salts are generally employed. Examples of sodium salts are sodium hexametaphosphate, sodium tripolyphosphate and tetrasodium pyrophosphate.

Suitable proportions of condensed phosphate salt vary for example with the origin and the processing of the clay and with the solids content of the slurry. Generally condensed phosphate dispersants are employed in amounts within the range of 0.1 percent to 0.5 percent of the clay weight.

It is within the scope of the invention. to employ a synthetic organic polyanionic dispersant along with a condensed phosphate salt to deflocculate the clay-.

water system.

In preparing coating colors, conventional adhesives or mixture of adhesives may be used. The adhesive functions to bind the clay particles to the substrate. Examples of adhesives are synthetic latices, e.g., styrenebutadiene, acrylics, starch (cooked or raw) and casein 4 EXAMPLE 1 In this example, the starting clay was a commercial sandground, pulverized acid grade of kaolin coating pigment supplied under the name Lamina". The clay was prepared by the grinding procedure described in U.S. Pat. No. 3.097.081 to Duke and was bleached with zinc hydrosulfite at an acidic pH, filtered and washed. The clay was about percent finer than 2 microns (equivalent spherical diameter).

One sample of the Lamina was processed, in accordance with the invention, as follows. To 106 grams water, 0.5 gram Bi(NO -,)5H O was added with mixing, producing a cloudy white suspension, indicating that the salt had hydrolyzed, forming insoluble bismuth oxynitrate. To the suspension Lamina was gradually added with mild agitation until the system thickened. Then tetrasodium pyrophosphate was added in small increments (about 0.05 percent based on the clay weight for eachincrement), followed by mixing, and incremental addition of more dry clay until a fluid clay suspension containing 65.5 percent clay solids was obtained. The suspension contained 0.45 percent tetrasodium pyrophosphate based on the dry clay weight. The pH was 5.7. Bismuth nitrate hydrate was present in amass! 9K rsrs msbsssst 9. 1 new. slay wei h A control suspension (66.9 percent solids) was prepared in conventional manner. The control had a pH of 6.2 and contained 0.34 percent tetrasodium pyrophosphate. This suspension was made down by conventional high shear mixing procedures.

Both suspensions had acceptable low shear and high shear viscosities.

Portions of the deflocculated suspensions were prepared into starch coating colors (49.9 percent solids) by mixing with PenFord Gum 280 in amount corresponding to' 17 parts by weight of the starch solids to 100 parts by weight clay solids. The pH of both coating colors was 7.0. The two coating colors had substantially the same Hercules end point viscosities. The Brookfield viscosities (10 rpm. and 100 rpm. with No. 4 spindle) of the experimental color were slightly-lower.

The colors were coated on the felt side of St. Regis basestock (24 lbs/ream 3300 ft?) to a coat weight of 4.9.1-191 ftinwt es ass .QflllQSQDIIQl nd 4.84..

TABLE EFFECT OF ADDITION OF BISMUTH NlTRATE TO COATING CLAY BEFORE MAKEDOWN ON THE OPTICAL PROPERTIES OF COATED PAPER Control & 0.25%

Control Bi(NO )'5H-,,O A*

Properties of Uncalendered Sheets I Brightness (Elrepho), 7r 68.] 69.1 l.0 B & L Opacity. '7: 88.9 89.8 0.9 Properties of Calendered Sheets 75 Gloss, 7r 32.7 I 33.6 0.9 B & L Opacity. '70 87.9 88.7 0.8 Brightness (Elrepho). 7r 66.8 67.7 0.9

' increase over control The adhesives are formed into aqueous suspensions or dispersions before they are mixed with the deflocculated clay slip containing the bismuth additive.

Conventional proportions of pigment and adhesive may be used. Thefollowing examples are given for illustrative purposes and are not considered to be limiting upon the invention.

Results are summarized in table form.

Opacity and brightness data in the table show that the addition of bismuth nitrate to the clay before makedown resulted in a significant increase in calendered and uncalendered brightness and opacity. 75 gloss was increased.

Portions of the control and experimental coating colors were coated side-by-side on lacquered, Lenata," black and white opacity charts to a thickness of 1% mil. Portions of the coatings were over black strips on board and others were over coated board. Visual observation of the coatings against both white and black backgrounds showed that the coatings made with the experimental clay were significantly more opaque. The pro cedure was repeated with heavier coatings (3 mil). In this case the contrast between the opacities was more striking and the experimental coatings were also visually whiter.

EXAMPLE ll The following example shows the benefit of treating an ultrafine hard kaolin clay pigment with a bismuth salt before makedown. The pigment was obtained from a hard gray kaolin clay crude from a mine near Mclntyre, Georgia. The crude was blunged in water, fractionated to about I00 percent minus 2 microns, deflocculated and beneficiated by froth flotation with an anionic collector. The flotation tailings were separated from the froth (a concentrate of yellow anatase impurity) and the tailings were thickened by adding acid to flocculate the clay and remove water from the flocs. The fiocs were then bleached by adding potassium permanganate solution and then zinc hydrosulfite solution, in the presence of sulfuric acid, as described in US. Pat. No. 3,353,668 to James B. Duke. The bleached beneficiated hard clay was filtered and washed.

A 56.0 percent solids slurry of the bleached beneficiated hard kaolin clay was prepared by agitating a portion of the acid filter cake with Bi(NO )'5l-l O, added as a solid, in amount of 0.50 percent of the dry weight of the clay.

The treated clay was prepared into a high solids (71 percent) slip by agitating it in water containing tetrasodium pyrophosphate in amount of 0.3 percent, based on the dry weight of the clay.

A 60 percent solids coating color was prepared by diluting the 71 percent solids dispersed slip with water, mixing and then adding a styrene-butadiene latex suspension (Dow 620 Latex) in amount corresponding to 18 parts by weight latex solids to 100 parts by weight clay solids. The pH of the coating color was 8.2.

A control coating color without any bismuth salt additive was prepared from predispersed hard clay obtained by slurrying another portion of the same filter cake in water containing tetrasodium pyrophosphate (based on the dry clay weight).

The control coating color and the experimental coating color containing clay treated with bismuth nitrate were separately coated on five black glass plates asdescribed in an article by G. A. Hemstock and R. J. Bergmam],v STUDIES OF RELATIONSHIPS BETWEEN SUSPENSlON AND PAPER COATING FlLM PROP- ER'IIES,"'TAPPI, November 1968, Vol. 51 I. No. l 1, pages 489 to 496. Gloss measurements were made with a Hunter Gloss Meter and brightness was obtained with an Elrepho Brightness Meter. Coat weight of the coating on each plate was determined.

From the value of the reflectance of the coating over a black body and the coat weight on the plate the scattering coefi'icient (s) was computed using the equation given at page 491 in the TAPPI article by Hemstock et al. (supra). Mean scattering coefficients were calculated from the s values for the five plates.

The mean scattering coefficient of the coatings prepared with predispersed hard clay containing 0.5 percent by weight bismuth nitrate additive was 0.14 ream/lb. The mean scattering coefficient of the control coatings was only 0.1 l ream/lb. A comparison of these values shows that the use of bismuth nitrate during clay processing resulted in a clay coating having a significantly greater scattering coefficient and thus more reflecting interfaces. Since opacity is evaluated quantitatively by scattering coefficient, the results indicate that the bismuth salt additive will markedly improve the opacity and hiding power of clay coatings.

I claim:

1. As a new article of manufacture, a kaolin clay pigment comprising a mixture of kaolin clay and a small amount of a bismuth +3 salt selected from the group consisting of nitrate, chloride, bromide, sulfate, carbonate and oxysalts thereof, the amount of said salt being sufficient to improve the opacity of coatings pigmented with said clay.

2. The article of claim 1 which also contains a condensed phosphate salt in amount sufficient to deflocculate said clay in water.

3. The article of claim 1 in which said bismuth salt is a nitrate salt.

4. In the wet processing of kaolin clay to provide a coating pigment wherein a suspension of a fine size fraction of degritted clay is bleached with zinc hydrosulfite at an acidic pH and the bleached clay is filtered, washed and fluidized by adding a sodium condensed phosphate salt, the improvement which comprises adding a hydrolyzable bismuth (+3) salt to the clay in amount within the range of 0.05 percent to 1 percent of the dry clay weight after filtering the clay and before adding said sodium condensed phosphate salt.

5. In the wet processing of kaolin clay to provide a coating pigment wherein a suspension of a fine size fraction of degritted clay is bleached and filtered at an acid pH and the resulting acidic clay filter cake is washed and fluidized by adding a clay dispersing agent, the improvement which comprises adding a hydrolyzed bismuth (+3) salt to the washed acidic clay filter cake before adding the dispersing agent, the bismuth salt being added in amount within the range of 0.05 to 1 percent of the dry clay weight. 

2. The article of claim 1 which also contains a condensed phosphate salt in amount sufficient to deflocculate said clay in water.
 3. The article of claim 1 in which said bismuth salt is a nitrate salt.
 4. In the wet processing of kaolin clay to provide a coating pigment wherein a suspension of a fine size fraction of degritted clay is bleached with zinc hydrosulfite at an acidic pH and the bleached clay is filtered, washed and fluidized by adding a sodium condensed phosphate salt, the improvement which comprises adding a hydrolyzable bismuth (+3) salt to the clay in amount within the range of 0.05 percent to 1 percent of the dry clay weight after filtering the clay and before adding said sodium condensed phosphate salt.
 5. In the wet processing of kaolin clay to provide a coating pigment wherein a suspension of a fine size fraction of degritted clay is bleached and filtered at an acid pH and the resulting acidic clay filter cake is washed and fluidized by adding a clay dispersing agent, the improvement which comprises adding a hydrolyzed bismuth (+3) salt to the washed acidic clay filter cake before adding the dispersing agent, the bismuth salt being added in amount within the range of 0.05 to 1 percent of the dry clay weight. 